1. Field of the Invention
This invention relates to drilling apparatus and in particular to expandable drilling or reaming apparatus.
2. Description of the Related Art
GB-A-2 320 270 describes an underreamer with extendable cutting blades. The tool may be run into a bore on a tubular drill string with the blades in a retracted position that is with the cutting face of each blade extending longitudinally of the tool body. On reaching the cutting location, an increase in fluid pressure within the body induces movement of a piston which acts, via appropriate cam faces, to pivot the blades through 90° to an extended cutting position.
In the retracted position, the blades lie adjacent and overlapping one another, within the circumference of the body. While this provides for a compact “running” arrangement, it limits the available cutting area of the blades and also prevents the blades from cutting while in the retracted position. Further, to permit full extension of the blades it is necessary for parts of the blade cutting faces to pass through a position just prior to the fully extended position, where the effective diameter described by the blades is larger than the bore gauge cut by the blades in the fully extended position. Thus, the blades must cut a profile in the bore wall to permit full extension. Similarly, on retraction of the blades following reaming of a section of bore, it is necessary for the blades to cut an enlarged diameter profile in the bore to permit blade retraction, or to pull the tool back up to the previously cut profile, before the blades may retract. Clearly, the requirement to cut such a profile is time-consuming and inconvenient, and is also impractical when the underreamer is located, for example, within steel casing.
Due to the 90° rotation of the blades from the retracted position to the extended position, a portion of the cam face on each blade is rotated to a position where the cam face forms a part of the cutting face of the blade, and thus is exposed to an increased risk of wear and damage, which could interfere with the subsequent successful retraction and extension of the blades.
It is among the objectives of embodiments of the present invention to obviate or mitigate these and other disadvantages of this and other prior art arrangements.